Tracking parameters allow analytics programs to differentiate between paid ads and other sources of traffic, such as direct visits, organic searches, or referrals. They are invaluable for understanding the effectiveness of online advertising.

From an SEO perspective, URL parameters can be somewhat problematic. People who click on your ads may like the content so much that they decide to share your webpage with friends or post it on their own websites. Although this type of viral marketing is a good thing, imagine what happens if the link that they copy looks something like this:

If this link is popular enough, then search engines will begin to index this URL. It may supplant the same version of this page without tracking parameters in the search results. Clicks on this new organic result will increment the visits number for Google(cpc) when it should really be attributed to Google(organic).

One popular SEO method of dealing with this challenge is instituting a 301 redirect and storing campaign variables in a cookie. Although this prevents search engines from indexing the same content multiple times, it has the undesired effect of stripping campaign parameters and generally messing up analytics tracking (boo!).

Google now supports a new format that provides webmasters with more control over the URL that is returned in the search results. You can specify your preferred version of the URL so that properties like link popularity are consolidated to this version. Unlike the 301 redirect, this will not affect your analytics tracking. Read more on how to set up your pages to avoid duplicate indexing on the Google Webmaster Central Blog.